The present invention relates to a method of separating and purifying a spent solvent discharged from a solvent extraction process in a nuclear fuel cycle, such as a reprocessing plant of spent nuclear fuel or nuclear fuel manufacturing plant.
The present invention can preferably be utilized in regeneration and disposal processes of such a spent solvent as described above.
A solvent prepared by diluting a phosphate, such as tributyl phosphate (TBP), and octylphenyl-N,N-diisobutylcarbamoyl methylphosphine oxide (CMPO), with a higher hydrocarbon, such as n-dodecane (hereinafter referred to simply as "dodecane") and kerosene, is widely used in a solvent extraction step of a reprocessing process of spent nuclear fuel or of wet recovery process of mixed-oxide fuel scrap in a nuclear fuel manufacturing plant.
The spent solvent generated in the solvent extraction step contains deterioration products, such as dibutyl phosphate (DBP), formed as a result of degradation of a portion of TBP by an acid, heat, radioactive rays, etc. Such deterioration products adversely affect the extraction when the spent solvent is recycled for reuse. Therefore, the deterioration products are removed by alkali washing with an aqueous solution of sodium hydroxide or sodium carbonate.
A radioactive waste containing the deterioration products thus removed, such as DBP, is converted into a vitrified solid or a bituminized solid by mixing the same with a vitrification additive or a bituminization additive. However, in order to stabilize a large amount of the sodium component incorporated by the alkali washing, it is necessary in this solidification treatment to use a large amount of these additives. Consequently, the development of a method of separating and purifying a spent solvent which enables deterioration products, such as DBP, to be removed from TBP without using any salts of sodium has been desired in the art.
On the other hand, methods such as vacuum freeze-drying and low-temperature vacuum distillation wherein difference in vapor pressure is utilized have been used as a method of separating TBP, DBP and dodecane from a spent solvent. However, they are disadvantageous in that the treatment capacity is small because of the low vapor pressure. Consequently, the development of a separation method having a large treatment capacity for a spent solvent has been desired in the art.
Moreover, when a spent solvent is heated under atmospheric pressure to conduct distillation into components, there occur problems involving the danger of fire or explosion and also the danger that volatile components undergo evaporation and sublimation upon heating, thus causing environmental contamination.
The applicant of the present invention has proposed a method (hereinafter referred to as "cooling crystallization method") of separating and purifying a spent solvent containing a phosphate and a higher hydrocarbon, which comprising exposing the spent solvent at a temperature not greater than the freezing point of the higher hydrocarbon but not less than the freezing point of the phosphate to selectively freeze the higher hydrocarbon, and separating a resulting frozen solid mainly composed of the higher hydrocarbon from a remaining solution containing the phosphate in a higher concentration (see U.S. Pat. No. 5,110,507 corresponding to Japanese Patent Laid-open Specification No. 3-293595(1991)).
However, the cooling crystallization method is not always satisfactory. This is because it is difficult to suitably control the temperature, cooling speed and other conditions in the course of the formation of the frozen solid, and TBP, DBP, etc., are incorporated into the frozen solid to form a solid/liquid mixture, whereby it becomes difficult to efficiently separate the higher hydrocarbon having a high purity. In addition, it is necessary to such a cryogenic temperature as -20.degree. C. or below for increasing the recovery of the higher hydrocarbon.